The present invention relates in general to dithiophosphates and, more particularly, it relates to dialkyl dithiophosphates which have a long-established utility as promoters or collectors in the beneficiation of mineral-bearing ores by flotation, most particularly copper ores.
Dialkyl dithiophosphates are defined by the following general formula: ##EQU1## WHERE R is an alkyl group generally having less than 6 carbon atoms, and Me is any suitable dissociating cation such as sodium potassium, calcium or the like. These compounds are produced by reacting phosphorus pentasulfide with aliphatic alcohols.
The compounds can be used in a variety of forms. Concentrated aqueous solutions of ammonium salts of dialkyl dithiophosphoric acids may be used, but these are unstable even with excess ammonium hydroxide, and decompose into the free acid and gaseous decomposition products in a short period. Neutral salts of alkyl diesters of dithiophosphoric acid have also been used, and while not corrosive, they are hard-to-dry gels or pastes that, once dry, are difficult to dissolve. A further approach was to neutralize the alkyl diesters with an alkali metal base, and heat the mixture with a large excess of the base to obtain a stable, dry solid, but this reduces the concentration of active ingredient and raises cost with no improvement in performance. Also, if the base is an alkaline carbonate this can cause precipitates to form in hard water, which hinders the flotation. Lastly, it has been proposed to provide an aqueous solution including a neutralized dithiophosphoric acid (or its hydrolysis product) with an excess of alkali metal base amounting to 10-25%.
These solutions are preferable to the pasty mixes including excess sodium carbonate, but stability for more than six months is questionable.
Early references to these compounds and their use as flotation promoters may be found in U.S. Pat. No. 1,593,232 and U.S. Pat. No. 2,038,400. Ammonium salt solutions of the compounds are disclosed in U.S. Pat. No. 2,206,284. Hydrolyzed compounds are disclosed in U.S. Pat. No. 2,919,025. The compounds stabilized with 10-25% excess alkali are disclosed in U.S. Pat. No. 3,086,653, and, most recently, certain specific branched-chain dithiophosphates are disclosed in U.S. Pat. No. 3,570,772.
It is believed that in systems containing 10-25% excess caustic, the excess caustic is slowly continuing to hydrolize the alkyl radicals from the phosphorus atom to form sodium phosphates, and there is a slow deterioration. Any solution of a salt of dialkyl dithiophosphate that obtains a pH of 6 or below becomes extremely corrosive and over several weeks forms polymers, hydrogen sulfide and other products of decomposition. For example, solutions of di-isobutyl dithiophosphate having a 20-25% excess of sodium hydroxide are observed to deposit a black, varnish-like sludge slowly with time. It is believed that the principle corrosive agent is the =P:SSH ion which attacks steel very rapidly. Since the only practical containers for the storage and shipping of these solutions are cold-rolled steel drums, this becomes an unsatisfactory approach. Further, the use of substantial excess caustic makes the solution dangerous to handle.